CHAUVIN, La. — Generations of shrimpers, crabbers and oystermen have set out from this bayou village to net their catch.

They share an emotional bond with Iowa’s farmers: Both harvest nature’s bounty to earn a livelihood. These fishermen depend on the sea, just as the nation’s top corn growers rely on the rich Midwest soil.

But there’s a key difference. Iowa farmers always know where they’ll find their crop. For those who work these waters, locating their harvest has become an increasingly taxing game of hide-and-seek.

Nitrates from the fertilizer and manure that Iowa’s farmers apply to their fields, mixed with sewage and runoff from suburban lawns, flow 800 miles down the Mississippi River to the Gulf of Mexico.